1. Technical Field
The invention relates to a method and apparatus for handling of calls for emergency services in telecommunications networks and, in particular, so-called intelligent networks.
The invention is also applicable to routing within an intelligent network of emergency services calls from mobile stations.
2. Background Art
In existing telecommunications networks, particularly in North America, telephone calls for emergency services are routed to a Public Service Access Point (PSAP) which is staffed by emergency services operators and, for reasons of reliability and continuity of service, is accessed by way of a tandem switch dedicated to such emergency services. To access emergency services, the caller dials the emergency services access code, which in North America is 9-1-1. When the end office detects the digits 9-1-1, instead of routing the call like a normal voice call, it routes the call immediately to the 9-1-1 tandem switch which routes the call to the Public Service Access Point. In order to determine the most appropriate emergency services centre to provide the required services, the emergency services operator will determine the location of the caller by first obtaining the calling line identifier, either automatically if the tandem switch is provisioned with Automatic Number Identification (ANI), or by questioning the caller, and using it to access an Automatic Location Identification database (ALI) and obtain the geographical location of the caller.
Although it is usual to provide a second ALI database as a "hot spare" to ensure reliability of service, this existing way of handling emergency services calls is not entirely satisfactory because, if the trunks between the end office and the dedicated tandem switch, or between the tandem switch and the PSAP, were blocked, that PSAP could not supply the necessary emergency service to that caller. A further disadvantage is that automatic location identification is not available for mobile users.
An object of the present invention is to overcome or at least mitigate these deficiencies of existing emergency services systems. To this end, the present invention provides for calls for emergency services to be alternate routed using out-of-band or common channel signalling.